1. Field of the Invention
The present invention relates to an inkjet printing apparatus and ink jet printing method equipped with a device that adjusts a drive timing independently for the forward movement of a print head and for the backward movement of the print head, and for individual printing areas.
2. Description of the Related Art
An ink jet type printing apparatus, (hereinafter referred to as an ink jet printing apparatus), prints by employing fluid inks and a print head that ejects ink droplets onto print media to form characters or images. Ink jet printing apparatuses perform high definition printing more easily than other printers and provide the additional advantages, such as high printing speeds, reduced operating noise and lower prices.
For image printing, an ink jet printing apparatus employs a print head to scan a print line on a sheet, a print medium, in a direction (the main scan direction) perpendicular to the direction in which the sheet is conveyed, and when one or a plurality of scans have been completed, the sheet is conveyed in a direction (the sub-scan direction) perpendicular to the main scan direction to the succeeding print line, and thereafter moves the print head along that print line. Then, according to the system that sequentially prints images on a print medium, when the print head travels one way (in the forward direction) and returns the other way (in the backward direction), printing (hereinafter, also called two-way printing) is performed in both directions to increase the printing speed.
However, unless the drive timings for the forward and backward movements of the print head are adjusted, an optimal image will not be printed. That is, printing positions on the print medium may be displaced between the forward movement and the backward movement of the print head, and in this case, density unevenness would appear in a printed image and cause deterioration of the image quality.
Therefore, for forward and backward printing, the print positions of the print head should be adjusted, or specifically, adjustment of the drive timing for the print head is required.
The print head is located opposite a support member (platen) that supports a print medium, such as paper or film, from the back, and in order to avoid any interference by the print medium supported by the platen, a predetermined gap (head-to-sheet distance) is defined between the print head and the surface of the print medium. When electrothermal transducing elements are driven based on print data that has been entered, the print head ejects ink droplets through ejection ports, and these ink droplets travel across the gap formed between the print head and the print medium and land on the surface of the print medium. In this manner, image printing can be performed.
When such an ink jet printing apparatus is employed to perform high-quality image printing, it is preferable that an appropriate setting be selected as a head-to-sheet distance (gap) between the print head and a print medium. Specifically, when the head-to-sheet distance between the print head and the print medium is too small, the possibility is increased that the print head will touch the print medium, and that either more ink than necessary will be attached to the print medium, or a print medium movement failure will occur. And when the head-to-sheet distance is too great, there is a possibility that the locations where ink droplets actually land will be displaced, from anticipated reference positions on the print medium, and accurate image printing will sometimes not be performed.
As a conventional well known countermeasure for this problem, an adjustment mechanism that adjusts the head-to-sheet distance between a print head and a platen is provided, so that the print head can always be adjusted to an appropriate position in accordance with the thickness of a print medium. Further, a printing mode is changed, i.e., for the previous printing mode, the top and bottom sides and right and left sides of a print medium were defined as non-printing areas (hereinafter also called “margined printing”), but these areas are defined as fully printable (hereinafter also called “marginless printing”).
For image printing performed by the ink jet printing apparatus, a print medium is sandwiched by two or more conveying rollers and is conveyed to a target print position, and the print head ejects ink while moving perpendicular to the direction in which the print medium is conveyed. However, in the portions close to the leading or trailing end of the print medium (the leading or trailing end portion of the print medium that is conveyed using only one conveying roller because of the mechanical configuration), the leading or trailing end might be warped, depending on the print medium type. In addition, when the marginless printing mode described above is performed to eject ink onto the leading and trailing ends, the possibility is increased that warping of the print medium will become greater.
The warping of the leading or trailing end of the print medium also adversely affects the printing quality. It is known that the distance between the print head and the print medium is reduced when the print medium is warped, and that during two-way printing, performed by the print head, deviations in the landing positions of ink have occurred between the forward and the backward movements of the print head.
Therefore, a mechanism that prevents deterioration of printing quality by the warping of the leading or trailing end of a print medium is disclosed in Japanese Patent Laid-Open No. 2005-305811. This mechanism extends the head-to-sheet distance for a case wherein a possibility exists that, when the print medium is positioned, the leading end portion is not held, and thus, the print medium may be warped and contact the print head, or reduces the head-to-sheet distance for a case wherein the leading end portion is held.
Further, a mechanism employed for correcting a deviation, accompanied by a change in the head-to-sheet distance, of landing positions provided by the reciprocating movement is disclosed in Japanese Patent Laid-Open No. 2005-144808. This mechanism changes a drive signal based on the amount of displacement between the target positions of pixels that should be formed by ejecting ink from the ink ejection ports and the actual locations of pixels that were formed by ejecting ink through the ink ejection ports.
Furthermore, a mechanism that corrects discrepancies in the landing positions of ink, due to the surface condition of a sheet, is disclosed in Japanese Patent Laid-Open No. 2006-159483. To perform this correction, the mechanism employs a head-to-sheet distance sensor provided upstream of a carriage for detecting the distance between a print head and a print medium located opposite, and employs the detection signal to control the timing for the switching on of a heater employed for the ejection of ink.
According to Japanese Patent Laid-Open No. 2005-305811, for the adjustment of the head-to-sheet distance, a print head in the middle of a printing area is electrically elevated or lowered. However, merely by moving the print head up and down, ink can not be caused to land in appropriate locations, in accordance with the degree of warping at the leading or trailing end of the print medium, and black or white stripes will appear, due to a deviation in the landing position, and deterioration of the printing quality will occur.
According to Japanese Patent Laid-Open No. 2005-144808, in the process for aligning the print positions between the forward and the backward movements of the ink jet print head, the drive timing for the forward scan is fixed, and only the drive timing for the backward scan is controlled. However, in this case, since the drive timing for the forward scan is fixed, ink may be ejected onto a location shifted away from the correct landing position for the forward scan.
Also, in Japanese Patent Laid-Open No. 2006-159483, a sensor is employed to measure the distance between the print head and a print medium in real time. However, because of the mounting of the sensor, it is anticipated that the manufacturing cost will be increased.
The present invention is provided to solve the above described shortcomings. A correction process is performed for deviations in the landing positions of ink, accompanied by a change in the head-to-sheet distance at the leading end portion of a print medium, between the forward movement and the backward movement of a print head, so that printing can be performed without deterioration of the printing quality.